


na ostří nože

by armethaumaturgy



Series: Left Hand AU [1]
Category: Undertale (Video Game)
Genre: Cross (X-tale) - Freeform, Ecto-Genitalia (Undertale), Explicit Sexual Content, Illustrations, Killer (Killertale) - Freeform, Killer/Cross - Freeform, Knifeplay, M/M, Marking, Orgasm Delay/Denial, Power Play, Vaginal Sex, Wet & Messy, kross - Freeform
Language: English
Status: Completed
Published: 2021-02-20
Updated: 2021-02-20
Packaged: 2021-03-16 18:16:08
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,247
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29580135
Author URL: https://archiveofourown.org/users/armethaumaturgy/pseuds/armethaumaturgy
Summary: Cross’ patience was wearing thin.The entire duration of the mission they’d been assigned together, Killer insisted on not listening to a word he’d said, despite being the mission captain. No, he’d had to go off on his own and created extra trouble for Cross to clean up. Not to mention all his mouthing off.Running his mouth, that’s what he did the best. It might have been time for Cross to teach him his place.
Relationships: Sans/Sans (Undertale)
Series: Left Hand AU [1]
Series URL: https://archiveofourown.org/series/2181261
Comments: 11
Kudos: 177





	na ostří nože

**Author's Note:**

> title taken from ewa farna's song of the same name.  
> this is an AU (lovingly called left hand) where i basically give cross self confidence :)  
> illustration is by my lovely wife, [@reclawedcat](https://twitter.com/reclawedcat) on twitter (18+) ♥ [Tweet link](https://twitter.com/reclawedcat/status/1363067193420705797?s=20)

Cross’ patience was wearing thin.

The entire duration of the mission they’d been assigned together, Killer insisted on not listening to a word he’d said, despite being the mission captain. No, he’d had to go off on his own and created extra trouble for Cross to clean up. Not to mention all his mouthing off.

Running his mouth, that’s what he did the best.

“Why so uppity, Cross?” he asked, with his ever-present leer, like he didn’t damn well know already. “We did what we were supposed to, didn’t we?”

“No thanks to you,” Cross snipped back. He could feel the frown tugging his browbones, but he didn’t maintain his usual, schooled expression. Killer wasn’t worth it.

“Sounds to me like someone needs to get laid.”

And really, that had been in, hadn’t it? Another one of Killer’s jabs, because he couldn’t control his running mouth, out of many. Belatedly, Cross thought he should’ve done something about them much earlier, but now was just as good of a time as ever, he supposed.

With a practiced ease, he twisted on a heel, the momentum of his body slamming Killer against the wall. His ulna dug into the space between two of Killer’s vertebrae, even through the turtleneck.

“You are insufferable,” Cross said, staring into those bottomless sockets. Even without eyelights, Killer somehow managed to look smug. “Listen here.”

Killer had gone lax in his hold, which was a little surprising. Cross had expected resistance, his free hand already flexing with the heat of magic, moments from summoning his weapon. But this would make it so much easier.

“You are nothing but a glorified serial killer. I’ve worked my ass off for my abilities, and my skill. I won’t have a loser like you telling me what to do, or refusing to listen to me. It’s time you learned your place.”

Killer laughed, a little choked as Cross kept putting pressure on his throat’s leylines, but clear nonetheless. “Buddy, I’m Nightmare’s right hand, I know my place damn well.”

Cross’ marrow boiled in his bones at the disrespect, but it didn’t show on his face. Slowly, he let himself smirk back, a lopsided thing that was sharp enough to cut. “Don’t tell me you don’t know?”

He wrenched one of Killer’s arms above his head, as far as it would go. In his inventory sat a length of rope he retrieved, much to the other’s visual surprise, and started winding around the wrist. Once he took his forearm off of his neck, Killer gasped in a sharp breath, but he still didn’t fight, not even as Cross yanked the other arm up, binding the other wrist to the one already there.

“Nightmare is left-handed,” he said, patting Killer’s flexing hands before testing the bonds, pulling the rope and watching it tighten itself just a bit further. “Now, you had better stayed still, if you value your limbs.”

Killer went deathly still, save for the miniscule twitching of his arm from the strain of being held up so far. Cross stepped away and held his arm out. The grin refused to leave his face, fueled by Killer’s sockets staring at him, wide and scared.

His knife fit into his palm like an extension of it, crimson glow illuminating the foyer in all its decadent glory until it faded away. He pointed the blade at the other skeleton, if only to watch his ribcage stutter under his clothes, and then pulled it back to drive it into the wall, cutting through layers of brick with ease.

He had to stop halfway through the blade’s length; any further, and he would’ve cut into Killer’s cranium. As it stood, there was an inch or so separating him from the sharpened edge, holding his arms up.

“Wasn’t that hard, was it?” he asked, summoning one of the regular knives. “I’d advise  _ staying  _ still.”

Angling his hand, Cross wedged the tip of his blade under the hem of Killer’s shirt and pulled up, ripping the fabric more than cutting it. Killer made a sound when the glinting edge came close to his SOUL, but Cross knew what he was doing. If he’d wanted to drive it into the oh-so-tempting target, he would have.

But Killer was right in one thing, at least; Nightmare did like him, and losing a team member, no matter how much he got on Cross’ nerves, wasn’t the best choice of action. Or maybe he was like Nightmare himself, with a small soft spot for the idiot, stars know why.

The black turtleneck fell down to reveal Killer’s ribcage, and, more importantly, the blood-red of his ecto-flesh, almost as bright as his SOUL itself. Cross’ eyelights flicked up to his skull, knife still held up by his neck.

Killer shrugged, as best he could in his position. “What can I say, I’m a helpful guy.”

Cross frowned, returning the knife down to his ribcage. A little flick cut a line on his sternum, just beneath where the SOUL floated. “Did I give you permission to speak? You are terribly insubordinate.”

“Mu—” Killer cut himself off with a gasp when Cross drew another line, intercrossing the first one to make a neat little X.

“Maybe now you will learn your place,” he said, conversationally, and dropped the knife down to the waistband of Killer’s shorts. They were tented, the glow showing very faintly through the fabric.

It didn’t take long to cut through those, either.

They fell in tatters around Killer’s ankles, and Cross looked him over. Aside from his cock twitching and his arms shifting minutely, he was staying still. Cross licked his teeth at the sight of the marrow dripping from the little wounds he’d made, slow and thick as it rolled over bone and then the tensing ecto-flesh of his stomach. Like it was trying to emulate the selfsame movement, Killer’s SOUL dripped at the bottom, edges looking frayed.

It flickered, until its shape shifted into one that more resembled a normal monster SOUL.

“You’re into this, aren’t you?”

Killer just shrugged in reply.

Cross decided to reward him, for being agreeable for once.

“Would you like anything?” he asked, tapping the flat side of the knife against his stomach. Killer was making a veritable mess of the floorboards under them.

“Yeah, for you to finally hu—”

“Ah, ah, ah,” Cross chided. He twisted the knife, so Killer could feel the blade pressing into the malleable flesh. “You address your superiors by ‘sir’. Beginning and end of sentence.”

Killer grit his teeth. They pulled up in a grin again, after a second, and he rolled his hips upwards. “Sir, hurry the fuck up. Sir.”

Cross growled, knife gone in favor of gripping Killer’s thigh and pulling it up. His claws dug into the flesh and there would no doubt be marks left, to remind Killer of this. “Don’t test me, or I will leave you here.”

“You fucking wouldn’t,” Killer scoffed. He backpedaled the moment Cross let go of his leg and let it fall back down, stepping away. “Sir! Fuck! Crossy, c’mon, I can’t explain this to Night when he comes around. Sir.”

“Maybe you should’ve thought of the consequences before opening your mouth. I’m willing to give you one more chance. Don’t squander it.”

Cross hoisted Killer’s leg up again, hooking it over his hip. “Sir, okay, sir,” Killer grit out, and it was good enough, for now.

Cross fumbled with his shorts with his free hand, shoving the waistband down enough to free his own cock. “You are free to speak only, and only if you want to stop. Understood?”

He lined up with Killer’s dripping entrance, but waited for an acknowledgement. The other skeleton wiggled, but stuck between the knife above and Cross’ hand holding him still, he couldn’t do much. But Cross was patient, content to wait until Killer addressed him properly.

Finally, Killer downright whined. “Sir, yes, sir.”

Cross’ hips snapped up. He halted himself in one thrust, gripping Killer’s hips if only to make sure he didn’t throw himself up into the knife. They hissed simultaneously as Killer’s tight walls spread for him. He was wet enough that there was no real pain, only a delicious friction.

“Fuck,” Killer sighed, ankles digging into the small of Cross’ back. 

Cross clucked his tongue. “Forgetting yourself already?” he asked, pulling back just so he could slam back in and pull another noise out of him.

“S-sir,” Killer amended, breathlessly. Cross set a pace, hard and fast. “Fuck, sir…”

Killer was tensing around him, walls like a velvety vice as they fluttered around his length, but Cross could still push it a bit further. He leaned in and smashed his teeth against Killer’s. It was less of a kiss than it was a mesh of teeth and tongues, but that suited them just fine.

“You will tell me when you’re close,” he ordered, and the look that passed Killer’s face was more than worth it. His eyelight — which had manifested at one point — narrowed into a pinprick. Idly, Cross wondered if that was too much, but Killer surprised him by meeting the next thrust.

“Sir, yes… sir…”

Each thrust pulled a shaky gasp or moan out of him, and Cross wasn’t much better, groaning into the space between them as Killer tightened around him. Some of them seemed deliberate, if Killer’s lazy grins were any indication, but most of them weren’t.

Cross took great pride at being able to reduce him to such a mess, drinking up every moan as it spilled past his teeth. And to think all it took was a couple knives and a wall. Maybe he really should’ve done this earlier.

He adjusted his grip on the other, and it changed the angle just enough to have Killer gasping and squeezing his sockets closed.

“Sir… m’close, sir,” he muttered, voice shaky.

His cock was leaking between them, and judging by the amount, Cross would have to wash his clothes. Since they were already dirtied, he pressed closer, as close as was physically possible. Killer’s dick ended up squished between them, along with his SOUL.

The bound skeleton hissed as the organ pressed against the still-bleeding cuts on his sternum. Their faces were close enough that they could feel each other’s breath.

“You’re not allowed to cum before I do,” Cross said, resuming his pace.

Killer jerked under him with a low whine. He clamped down on Cross with a mutter of something that Cross decided to let him off the hook for, and then started meeting the pace again, doubling his effort in bringing Cross close.

And Cross was loath to admit it was working. He could barely control his hips; every time he pulled out, Killer’s walls held him, like they didn’t want to let him go, and every time he slammed back in, they’d flutter just a little. Pleasure sparked along his spine as his magic wound tighter and tighter, a coil in his pelvis that just begged to snap.

He pushed in as far as he could, pushing against the end of Killer’s passage. He bit out a moan, pace breaking as hips stuttered in tiny, shallow thrusts. He muffled himself in Killer’s neck, teeth biting down on the sensitive vertebrae as he spilled himself, painting Killer’s insides with purple.

It spilled past his length, adding to the mess already on the floor. Killer’s whole body shook, hips twitching even as Cross stilled.

“S-sir… c’mon, Crossy, m’close,” he babbled, rubbing inside against Cross’ shirt. Taking pity on him, Cross wormed a hand between them and wrapped it around his cock, jerking it hard and fast. “Fuck… fuck, yeah, like that—! Fuck!”

If Cross was mean, he would’ve counted just how little it took to have Killer cumming all over his hand. But he wasn’t, so he didn’t say anything as he wiped the red residue into the tatters of Killer’s shirt.

It’d been seven pumps.

He pulled away and righted his own clothes. While yes, they definitely needed a good wash, but compared to Killer, who was a mess of fluids and the only article on him that wasn’t completely ruined was his hoodie.

Killer slumped down, all but hanging off the knife, the only thing holding him upright, knees shaky and ribcage shaking with his labored breathing. Cross found it a good look on him, in all honesty.

“I’m going ahead to my room. Make sure to report to Nightmare about our mission,” he said, turning on his heel to head down the long hallway leading off to their personal chambers.

“W-wait! Cross!” Killer called after him, so he paused, fighting a smirk that threatened to split his face. “You can’t leave me here! Uh… sir.”

Cross lost the battle with the smirk, and he let Killer see  _ just  _ how wide it was. “Say it again. Properly.”

Killer wiggled in place, and Cross took a bit of sadistic pleasure in the knowledge that Killer was no doubt feeling all the cum cooling on him. “Sir. Let me down, sir.”

Cross released the magic and the knife in the wall dissipated in a sprinkle of crimson particles. Like a marionette with its strings cut, Killer fell to the ground, right into the puddle of slick he’d left there. Cross caught the tail end of some quiet mumbles, but by then, he was already turned away again.

“Good job. I look forward to the next time you misbehave.”


End file.
